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We report a combined approach of stationary and time-resolved fluorescence measurements and ultraviolet-visible (UV-vis) transient absorption spectroscopy (TAS) along with ab initio calculations, which provide an overall picture of the dynamics occurring after excitation in a push-pull molecule, namely, 4,7-bis (4,5-dibutylbenzo[1,2-b:4,3-b?]bisthiophene[1,2-b:4,3-b?]bisthiophen-2-yl)-2,1,3-benzothiadiazole. The analysis of the emission spectra in solvents of different polarities reveals the presence of three conformers whose structures differ in the orientation of the 4,5-dibutylbenzo-bisthiophene groups and in their planarity with respect to the benzothiadiazole acceptor group. The Kawski method allows us to estimate the ground- and first-excited state dipole moments (? g and ? e ) for the three conformers. We find values of ? e similar for the three conformers and higher than the relative ? g values as can be expected from a push-pull molecule undergoing a light-induced charge-transfer (CT) transition. UV-vis TAS in different solvents highlights the instantaneous (within our instrumental resolution) formation of a locally excited S 1 state (accompanied by a big change in the dipole moment with respect to S 0 ), which undergoes a rapid intramolecular CT (ICT) assisted by molecule planarization [planar ICT (PICT)]. The strong dipole-dipole interactions with the polarized solvent molecules stabilize the S 1 CT state that decays principally through fluorescence emission. Both PICT and solvation dynamics are responsible for the big Stokes' shift characterizing the molecule, particularly in polar solvents. The fluorescence lifetimes are substantially longer in polar solvents, and also fluorescence quantum yields are higher in polar solvents. We conclude that the radiative relaxation time increases when molecular planarization of the S 1 emissive state takes place, and this condition is favored in polar solvents where local dipole-dipole interactions support the structural stabilization of the CT emissive state. In the poly(methyl methacrylate) matrix, the structural and solvation dynamics are strongly inhibited, leading to reduction of nonradiative processes and to shortening of the fluorescence relaxation time

A series of Bodipy dimers with orthogonal conformation were prepared. The photophysical properties were studied with steady-state and time-resolved transient spectroscopies. We found the triplet-state quantum yield is highly dependent on the solvent polarity in the orthogonally linked symmetric Bodipy dimers, and the intersystem crossing (ISC) is efficient in solvents with moderate polarity. The photoinduced symmetry-breaking charge transfer (SBCT) in polar solvents was confirmed by femtosecond transient absorption spectroscopy, with the charge separation (CS) kinetics on the order of a few picoseconds and the charge recombination (CR) process occurring on the nanosecond time scale in dichloromethane. These observations are supported by the calculation of the charge separated state (CSS) energy levels, which are high in nonpolar solvents, and lower in polar solvents, thus the CR-induced ISC has the largest driven force in solvents with moderate polarity. These results clarify the mechanism of SOCT-ISC in the orthogonally symmetric Bodipy dimers. The acquired information, relating molecular structure and ISC property, will be useful for devising new strategies to induce ISC in heavy atom-free organic chromophores.

The photophysics of 9(19),16(17),23(24)-tri-tert-butyl-2-[ethynyl-(4-carboxymethyl)phenyl] phthalocyaninatozinc(II) (ZnPc) in solution and adsorbed on TiO2 and ZrO2 nanoparticle films is characterized by stationary and time-resolved spectroscopies in the subpicosecond to nanosecond time interval. The comparison between the solution and the solid substrate data allows us to identify different pathways of the energy and electron relaxation. On the solid substrate, the presence of H-aggregates adds a further nonradiative deactivation channel competing with the charge injection into the Ti-2-conducting band, thus providing an explanation of the reduced efficiency of the charge transfer processes. The comparison between the kinetics recorded after excitation of the S-0-S-2 transition and those recorded after excitation of the S-0-S-1 transition provides an estimate of the internal conversion between S-2 and S-1 which occurs very efficiently and on an ultrafast (